Compensation of radio system RF losses, and in particular compensation of radio system RF losses using closed loop gain compensation.
In radio installations generally, the amount of radio frequency (RF) energy transmitted at the antenna is desirably held consistent from one installation to another. However, many sources of variation in each device result in significant variations. In ground-based single channel communication, the satellite accounts for variations by transmitting command signals to the ground-based unit to increase or decrease power output during transmission. Multichannel communication system installations for use in mass transportation vehicles, such as commercial air transport aircraft, are more complex.
Multichannel communication systems accept data and voice from various sources onboard a vehicle, encode and modulate this information to appropriate RF carrier frequencies, and transmit these carriers over any of multiple transmission channels to the satellite constellation for relay to the ground. Multichannel satellite communication (SatCom) systems also receive RF signals from the satellite constellation, demodulate these signals, perform the necessary decoding of the encoded messages, and output data or voice for use onboard the vehicle by crew members and passengers. Transceivers in such multichannel mobile satellite communication systems include a main system CPU for performing the actual transmit and receive functions, a radio control subsystem that allocates transmission channels to calls, a high power amplifier for boosting the channel power, a common antenna receiving and transmitting signals, and a low noise amplifier amplifying the RF signal received from a satellite. In multichannel mobile communication systems, such as an aircraft installation, many sources of variation in each installation result in significant installation-to-installation variations. For example, typical aeronautical SatCom system installations divide the system functions into multiple separate modules, including a telecommunications module housing the main system CPU and the radio control subsystem, a high power amplifier module, a low noise amplifier module, and the antenna. One important source of variation is inconsistencies in the equipment manufacture. Another important source of variation is the use of different types and lengths of wiring, usually coaxial cable, to interconnect the various physically separated modules, or components, of the communication unit. Although the various functional modules are interconnected with standardized wires or cables for inter-module control and to connect RF signals, installation-to-installation cable type and length variations produce variations in the amount of RF energy at the antenna.
While the desirability of holding the amount of RF energy transmitted at the antenna consistent from one aircraft installation to another is recognized, the necessary use of different types and lengths of cable in different aircraft installations is overcome only by a universal standard cable type and length. Such a standard cable is necessarily the cable required for the most demanding application. Thus, installation-to-installation consistency would require every aircraft to carry the longest, heaviest coupling cables. However, in aircraft installations, the addition of excess cable length and weight is not desirable. Furthermore, a universal type of cable may not satisfy the requirements of all radio installations. Therefore, the variations must be compensated in another way.
The satellite attempts to account for these and other variations by transmitting command signals to the communication unit to increase or decrease power output during transmission. In multichannel aircraft installations, the radio control subsystem of the communication unit dynamically controls the output power for each radio transmission channel. The typical communication unit uses closed loop power control algorithms, such as an automatic gain control circuit, for controlling RF power levels at the antenna. The transmitter communication unit receives transmit power level commands from the network satellite, which are intended to control the amount of power radiated by the antenna. The automatic gain control circuit causes the radio control subsystem to increase or decrease power output on each active radio transmission channel in response to command signals transmitted from the satellite. However, the changes in output power applied by the radio control subsystem are not translated consistently into output power at the antenna because the differing amounts of power absorption by the RF cables interconnecting the various modules results in variations in the coupling losses between the radio control subsystem and the antenna which cannot be compensated by the automatic gain control circuit. Such losses may range anywhere from 0 to 20 dB or more, depending on the installation.
Thus, even with standardized intermodule wiring, each installation results in different amount of cable loss relative to other similarly wired installations. This variation in RF cable loss presents problems with the closed loop power control algorithms of many second generation satellite systems. The installation-to-installation differences in the amount of RF cable loss causes variations in the amount of RF energy at the antenna. Thus, these installation-to-installation variations in cable loss produce variations in the amount of power radiated by the antenna.
Manual control of the radiated power variations are impractical. For example, attempting to reduce the installation-to-installation variation by tightly controlling the cable types and cable lengths results in a significantly more difficult installation. Manually measuring power levels and manually adjusting the gain of the high power amplifier until a specified power level is measured also results in a significantly more difficult installation.
Furthermore, in installations where standard cabling is provided, adding a cable for a new purpose, such as determining the output power at the antenna relative to the output power at the transmission channel, is not an practical option. Therefore, the detection and communication of system losses must utilize existing cables. One attempt to resolve the coupling losses between the radio control subsystem and the antenna added a DC bias on the return cable from the antenna to the radio control subsystem. However, the DC bias is subject to the same cable losses as the original signal.
Therefore, what is needed is a means for accurately determining the output power at the antenna relative to the output power at the transmission channel without additional cabling.
The present invention overcomes the limitations of the prior art by providing a method and circuit for automatically compensating the installation-to-installation cable loss variation without manual intervention, thereby providing easier and less expensive radio installations. Furthermore, the method and circuit of the present invention continually compensates the radio system power level variations, resulting in a radio system that is robust to radio system variations, such as variations in gain of the radio system high power amplifier due to manufacturing variations, temperature fluctuations, and other factors affecting cable loss.
The method and circuit of the present invention provide means for dynamically adjusting the output power at the antenna of a radio system by determining the power level difference between the signal source and the system antenna, and dynamically adjusting the system gain in response to the detected power level difference.
In any radio installation wherein system RF losses can be compensated by adjusting the gain, the present invention provides a closed loop gain compensation method utilizing preexisting cabling within the radio system.
The present invention overcomes the gain compensation limitations in radio transmission and reception systems of the prior art by providing an automatic gain compensation circuit for a radio transmission and reception system formed of a circuit coupled to receive a transmission signal generated by a radio system transmitter, the circuit generating an output power signal, which is representative of the transmission signal output power, and transmitting the signal on the radio system""s receiver line. The automatic gain compensation circuit of the invention also includes a power control circuit coupled to receive the output power signal, which generates a power control signal in response to the output power signal and communicates the power control signal to the radio system signal amplifier using the existing transmission and reception lines, i.e., without change to the existing installation. Preferably, the output power signal is either a sinusoidal signal proportional to the radio system""s transmission signal output power, or a frequency modulated signal with digital information representative of the transmission signal output power contained in the modulation. The transmission signal generated by a radio system received by the circuit is preferably representative of the radio system output power at the radio system""s antenna.
According to one aspect of the invention, the power control signal generated by the circuit is a digital signal transmitted at a frequency different from the radio system transmission signal frequency. For example, in a radio system transmitting in the L-band, the digital signal generated by the circuit is modulated in the frequency range of about 1 kHz to 5 kHz. Alternatively, the power control signal generated by the circuit is a sinusoidal signal having a frequency range of about 1 kHz to 5 kHz.
According to another aspect of the invention, the gain compensation circuit includes a RF power level detector coupled to detect the power level in the RF signal generated by a radio system and generates a voltage signal proportional to the transmission signal output power. A converter coupled to receive said voltage signal converts the voltage signal into a frequency signal representative of the output voltage signal. A signal summer coupled to receive both the representative frequency signal and a RF signal received by the radio system combines both signals into a single combined signal for transmission on the radio system receiver cable to a signal splitter. The signal splitter splits the signals and outputs both the representative frequency signal and the received RF signal as separate outputs. A power controller coupled to receive the representative frequency signal from the signal splitter generates a power control signal and outputs the power control signal to the radio system""s signal amplifier.
According to other aspects of the invention, the invention overcomes the limitations of the prior art by providing a method implemented in the automatic gain compensation circuit of the invention, the method including automatically determining the RF power losses of a radio frequency transmission system; communicating the losses to the portion of the radio frequency transmission system originating the radio frequency transmission using preexisting cabling within the radio transmission system; and controlling the RF power gains to compensate for the losses. The method preferably includes detecting a RF power level of the radio frequency transmission; generating a signal indicative of the RF power level of the radio frequency transmission; and transmitting the indication signal to the portion of the radio transmission system originating the radio frequency transmission using preexisting cabling within the radio transmission system. Transmitting the indication signal includes combining the indication signal with a RF signal originating externally to the radio transmission system and received thereby, and transmitting the two signals together on the radio system""s receiver cable.
According to one aspect of the method of the invention, the method also includes generating a power control signal and communicating the power control signal to the portion of the radio frequency transmission system controlling RF power gains of the radio frequency transmission to control the RF power gains to compensate for losses in the radio system.
According to another aspect of the invention, the automatic gain compensation circuit of the invention is implemented in a radio transmission and reception system having a transmitter generating a RF signal and a high power amplifier coupled to the transmitter by a first transmitter cable, the high power amplifier dynamically increasing and decreasing the power of the RF signal in response to the power control signal. The amplified RF signal is output via a second transmit cable to the circuit of the invention coupled to receive the transmission signal generated by the radio system, the circuit including a diplexer that detects the power level of the internally generated RF signal.
According to another aspect of the invention, a radio system RF antenna is coupled to the diplexer by a receiver/transmitter cable to receive the internally generated RF signal and transmit a RF transmission signal representative thereof. The antenna also receives a RF signal originating outside of the radio system, for example, by a satellite, and outputs the received RF signal. Preferably, a low noise amplifier is coupled to the antenna by the receiver/transmitter cable to receive the antenna output, i.e., the received RF signal. The low noise amplifier amplifies the received RF signal as required by the radio system.
According to another aspect of the invention, a signal summer is coupled to the diplexer to receive the output power signal and is also coupled to the low noise amplifier to receive the received RF signal. The signal summer sums the two signals into a single combined signal and outputs the combined signal on the radio system""s receiver line.
According to still another aspect of the invention, the power control circuit includes a signal splitter coupled to the radio system""s receiver line to receive the combined signal. The signal splitter divides the combined signal into its component parts: the representative output power signal and the received RF signal. The power control circuit also includes a power control signal generator coupled to receive the representative output power signal from the signal splitter and generate a variable power control signal responsive to the output power signal. The power control signal generator is coupled to communicate the variable power control signal to the high power amplifier. In a preferred radio system, a receiver is coupled to the signal splitter for receiving the received RF signal.
According to other aspects of the invention, the invention provides the method for automatic gain compensation in a radio system wherein the automatic gain compensation function utilizes the cabling preexisting within the radio system.